Slide 1

Soil Mechanics

Topic :Factors influencing compaction of soil and influence of compaction on properties of soil as well as field compaction methods PREPARED BY :)

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Factors affecting compaction of soil

The important factors affecting compaction are

Water ContentAmount of CompactionType of soilMethod of compactionThickness of layerSaturation lineAdmixturesStone content

Water content: At low water content, the soil is stiff and offers more resistance to compaction.As the water content is increased the soil particles get lubricated.The soil mass becomes more workable and the particles have closer packing.Admixtures: The compaction characteristics of the soils are improved by adding some admixtures.The most commonly used admixtures are lime, cement and bitumen.Stone content: With addition of aggregates of 20 to 30 mm size up to 40% volume, the density of soil increases.Method of compaction: The dry density achieved also depends upon the method of compaction.For the same amount of compactive effort, the dry density will depend upon whether the method of compaction utilizes kneading action, dynamic action.

Thickness of layer: In the compaction of soils the thickness of soil layer also play an important role.Normally the thickness of layer in the laboratory is kept 30 to 40 mm, while in the field the thickness of soil layer is kept about 200 to 300mm. The lesser thickness of soil layer gives higher dry density.Saturation line: If all the air voids are removed from the soil by compaction, the soil becomes fully saturated and higher max dry density is achieved.Saturation below 100% results in lower max dry density.

Type of soil : The maximum dry density achieved depends upon the soil.Coarse grained soil can be compacted to higher dry density than fine grained soils.A well graded sand attains a much higher dry density than a poorly graded soil .Cohesive soils have high air voids.

Methods of field compaction

The various method of field compaction

Rolling

Ramming

Vibrations

RollersDifferent types of rollers are used for compaction of soils.The compaction depends upon the following factors.Contact pressureNumber of passes (normally 5 to 15)Layer of thickness (less than 15cm)Speed of rollerDifferent types of rollers used areSmooth wheel rollersPneumatic tyred rollers sheep foot rollersTrack laying vehicles

RammingA hand operated rammer consists of a block of iron, about 3 to 5 kg in mass attached to a wooden rod.The rammer is lifted for about 0.30m and dropped on the soil to be compacted.Tampers are used to compact soils adjacent to existing structures such as trenches, within plinth of building etc.

VibrationsIn vibratory compactors, vibrations are induced in the soil during compaction. The compactors are available in a variety of forms.When the vibrator is mounted on a drum, it is called a vibratory roller.These rollers are available both as pneumatic type and the smooth wheel type.Another form of vibratory compactor is a vibrating plate compactor.Hand operated vibrating plates are also available. The effect of the vibrating plates is limited to small depths

Different types of rollersSheep foot Rollers: The sheep foot roller consists of a hollow drum with a large number of small projection on its surface.These projections penetrate the soil layers during the rolling operations and cause compaction.The drum can be filled with water to increase the mass.The sheep-foot rollers are ideally suited for compaction of cohesive soils.

Vibratory rollers: The rollers are available both as pneumatic type and smooth wheel type.In a smooth wheel type, as separate motor drives an arrangement of eccentric weight to create high frequency, low amplitude, up and down oscillations of the drum.These rollers are suitable for compacting granular soils.In a pneumatic tyred vibratory compactor, a separate vibratory unit is attached to the wheel axle.

Pneumatic tyred Rollers: Pneumatic tyred Rollers use compressed air to develop the required inflation pressure.The roller generally consists of 9 to 11 wheels fixed on two axles, with the pneumatic tyres so spaced that a complete coverage is obtained with each pass of the roller.These rollers are effective for compacting cohesive as well as cohensionless soil.

Smooth wheel rollers

Types of smooth wheel rollers

Conventional three wheel type having weight 2 to 15 ton.Tendem rollers weight having 1 to 14 ton.Three axle tendem roller having 12 to 18 ton.

These rollers are useful for compacting granular base courses of highway and runways.These are not effective for compaction of deep layers of soils.

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